A、By categorizing the world’s climate. B、By defining the term "climate. " C、By summarizing the previous lecture. D、By referring

Today I want to talk about the Earth’s last major climatic shift, at the end of the last ice age.
    But first, let’s back up a moment and review what we know about climatic change in general. First, we defined "climate" as consistent patterns of weather over significant periods of time.
    In general, changes in climate occur when the energy balance of the Earth is disturbed. Solar emergy enters the Earth’s atmosphere as light and is radiated by the Earth’s surface as heat. Land, water, and ice each affect this energy exchange differently. The system is so complex that, to date, our best computer models are only crude approximations and are not sophisticated enough to test hypotheses about the causes of climatic change.
    Of course, that doesn’t keep us from speculating. For instance, volcanic activity is one mechanism that might affect climatic change. When large volcanoes erupt, they disperse tons of particles into the upper atmosphere, where the particles then reflect light. Since less light is entering the system of energy exchange, the result would be a cooling of the Earth’s surface.
    Of course, this is just one possible mechanism of global climate change. In all probability, a complete explanation would involve several different mechanisms operating at the same time.
10. How does the speaker begin her discussion?
11. What does the speaker mean by the phrase"the energy balance of the Earth?"
12. Why do meteorologists have difficulty testing hypotheses about climatic changes?